For internal combustion engines, such as diesel engines, nitrogen oxide (NOx) compounds may be emitted in the exhaust. To reduce NOx emissions, a SCR process may be implemented to convert the NOx compounds into more neutral compounds, such as diatomic nitrogen, water, or carbon dioxide, with the aid of a catalyst and a reductant. The catalyst may be included in a catalyst chamber of an exhaust system, such as that of a vehicle or power generation unit. A reductant, such as anhydrous ammonia, aqueous ammonia, or urea is typically introduced into the exhaust gas flow prior to the catalyst chamber. To introduce the reductant into the exhaust gas flow for the SCR process, an SCR system may dose or otherwise introduce the reductant through a dosing module that vaporizes or sprays the reductant into an exhaust pipe of the exhaust system up-stream of the catalyst chamber.
In operation, the injected reductant and exhaust gas typically mix in a decomposition reaction chamber or tube such that the injected reductant can be processed into ammonia and mix with the NOx of the exhaust gas. To maximize reduction of NOx emissions by the SCR catalyst, a substantially constant ammonia to NOx ratio (ANR) distribution throughout a cross-section prior to the SCR catalyst may be preferred. Depending on how the reductant is dosed, the configuration of a dosing module and/or decomposition reaction chamber or tube, and/or other factors, the resulting exhaust gas-reductant composition can vary along the exhaust system and at cross-sections of the exhaust system. This may result in a variable ANR distribution along the exhaust system and at cross-sections of the exhaust system.